Do not initialize CMAKE_BUILD_TYPE on multi-config generators

Use the `GENERATOR_IS_MULTI_CONFIG` global property to reliably
detect multi-config generators regardless of what variables the
project or user have set.

Modules/CMakeASMInformation.cmake

@@ -75,11 +75,13 @@ set (CMAKE_ASM${ASM_DIALECT}_FLAGS "${CMAKE_ASM${ASM_DIALECT}_FLAGS_INIT}" CACHE
      "Flags used by the assembler during all build types.")
 
 if(NOT CMAKE_NOT_USING_CONFIG_FLAGS)
-# default build type is none
-  if(NOT CMAKE_NO_BUILD_TYPE)
+  get_property(_GENERATOR_IS_MULTI_CONFIG GLOBAL PROPERTY GENERATOR_IS_MULTI_CONFIG)
+  # default build type is none
+  if(NOT _GENERATOR_IS_MULTI_CONFIG AND NOT CMAKE_NO_BUILD_TYPE)
     set (CMAKE_BUILD_TYPE ${CMAKE_BUILD_TYPE_INIT} CACHE STRING
       "Choose the type of build, options are: None, Debug Release RelWithDebInfo MinSizeRel.")
   endif()
+  unset(_GENERATOR_IS_MULTI_CONFIG)
   set (CMAKE_ASM${ASM_DIALECT}_FLAGS_DEBUG "${CMAKE_ASM${ASM_DIALECT}_FLAGS_DEBUG_INIT}" CACHE STRING
     "Flags used by the assembler during debug builds.")
   set (CMAKE_ASM${ASM_DIALECT}_FLAGS_MINSIZEREL "${CMAKE_ASM${ASM_DIALECT}_FLAGS_MINSIZEREL_INIT}" CACHE STRING

Modules/CMakeCInformation.cmake

@@ -110,11 +110,13 @@ set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS_INIT}" CACHE STRING
      "Flags used by the compiler during all build types.")
 
 if(NOT CMAKE_NOT_USING_CONFIG_FLAGS)
-# default build type is none
-  if(NOT CMAKE_NO_BUILD_TYPE)
+  get_property(_GENERATOR_IS_MULTI_CONFIG GLOBAL PROPERTY GENERATOR_IS_MULTI_CONFIG)
+  # default build type is none
+  if(NOT _GENERATOR_IS_MULTI_CONFIG AND NOT CMAKE_NO_BUILD_TYPE)
     set (CMAKE_BUILD_TYPE ${CMAKE_BUILD_TYPE_INIT} CACHE STRING
       "Choose the type of build, options are: None(CMAKE_CXX_FLAGS or CMAKE_C_FLAGS used) Debug Release RelWithDebInfo MinSizeRel.")
   endif()
+  unset(_GENERATOR_IS_MULTI_CONFIG)
   set (CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG_INIT}" CACHE STRING
     "Flags used by the compiler during debug builds.")
   set (CMAKE_C_FLAGS_MINSIZEREL "${CMAKE_C_FLAGS_MINSIZEREL_INIT}" CACHE STRING

Modules/CMakeCommonLanguageInclude.cmake

@@ -17,11 +17,13 @@ foreach(t EXE SHARED MODULE STATIC)
 endforeach()
 
 if(NOT CMAKE_NOT_USING_CONFIG_FLAGS)
-# default build type is none
-  if(NOT CMAKE_NO_BUILD_TYPE)
+  get_property(_GENERATOR_IS_MULTI_CONFIG GLOBAL PROPERTY GENERATOR_IS_MULTI_CONFIG)
+  # default build type is none
+  if(NOT _GENERATOR_IS_MULTI_CONFIG AND NOT CMAKE_NO_BUILD_TYPE)
     set (CMAKE_BUILD_TYPE ${CMAKE_BUILD_TYPE_INIT} CACHE STRING
       "Choose the type of build, options are: None(CMAKE_CXX_FLAGS or CMAKE_C_FLAGS used) Debug Release RelWithDebInfo MinSizeRel.")
   endif()
+  unset(_GENERATOR_IS_MULTI_CONFIG)
 
   set (CMAKE_EXE_LINKER_FLAGS_DEBUG ${CMAKE_EXE_LINKER_FLAGS_DEBUG_INIT} CACHE STRING
      "Flags used by the linker during debug builds.")